The present invention relates generally to cardiac pacemakers. More particularly, the present invention relates to cardiac pacemakers having improved methods for detecting arrhythmias.
An arrhythmia is a heart rhythm disorder which interferes with the life sustaining blood pumping action of the heart. Examples of arrhythmias include ventricular tachycardia and atrial tachycardia. Ventricular tachycardia effects the lower chambers of the heart, the ventricles, and atrial tachycardia effects the upper chambers of the heart, the atria. Ventricular tachycardia is a rapid heart beat initiated within the ventricles, characterized by three or more consecutive premature ventricular beats. Ventricular tachycardia is a potentially lethal arrhythmia, as it may cause the heart to become unable to pump adequate blood through the body. Companies such as Medtronic, Inc., have developed implantable pacemakers which may be used to successfully treat ventricular tachycardia by delivering ventricular pacing pulses to the heart when ventricular tachycardia is detected.
Dual chamber pacing modes have been widely adopted for pacing therapy. Among the dual chamber operating modes is the xe2x80x9cDDDxe2x80x9d mode, which can pace an atrium and a ventricle, senses both the atrium and the ventricle, and can either inhibit or trigger pacing stimuli for both chambers. This mode has a sensor augmented variant mode called xe2x80x9cDDDRxe2x80x9d, where the xe2x80x9cRxe2x80x9d stands for rate-adaptive or rate modulation.
A DDD pacemaker includes an atrial sense amplifier to detect atrial depolarizations of the heart, and a ventricular sense amplifier to detect ventricular depolarizations of the heart. If the atrium of the heart fails to beat within a predefined time interval (atrial escape interval), the pacemaker supplies an atrial stimulus to the atrium through an appropriate lead system. Following an atrial event (either sensed or paced) and an atrioventricular (A-V or A2V) interval, the pacemaker supplies a ventricular pacing stimulus to the ventricle through an appropriate lead system, if the ventricle fails to depolarize on its own. Pacemakers which perform this function have the capability of tracking the patient""s natural sinus rhythm and preserving the hemodynamic contribution of the atrial contraction over a wide range of heart rates.
Various types of pacemakers are disclosed in the prior art, and are presently in widespread use. The pacing literature has documented the different types of pacemakers and their characteristics extensively. A summary of the evolution and characteristics of pacemaker types, and specifically different types of dual chamber pacemakers, is set forth in U.S. Pat. No. 4,951,667, which is incorporated herein by reference.
Another and more recent advance in the field of cardiac pacing systems is that of the rate responsive pacemaker which increases cardiac output in response to exercise or other body demands. Such pacemakers may control pacing rate based upon sensing any one or a combination of different body parameters such as body activity, blood pH, respiratory rate, QT interval or historical atrial activity. See, for example, U.S. Pat. No. 4,428,378, (Anderson et al.), disclosing a pacemaker which varies pacing rate in response to sensed patient activity; and U.S. Pat. No. 4,228,308, (Rickards), which discloses controlling pacing rate in response to Q-T interval. Additionally, rate responsive control has been integrated into dual chamber pacing systems, e.g., DDDR and DDIR systems. See xe2x80x9cRate Responsive Dual Chamber Pacingxe2x80x9d in PACE, vol. 9, pp. 987-991; U.S. Pat. No. 4,467,807, Bornzin; and the above-noted U.S. Pat. No. 4,951,667.
Background information directly related to the present invention may be discussed in greater detail. The atrium may be paced with an A-pace. The energy from the A-pace may be sensed by the ventricle amplifier as a V-sense event. This is referred to as an over-sense or cross-chamber sensing. It is not really a contraction of the ventricle, but is rather the electrical activity of the atrium being detected by the sensor in the ventricle. In this situation, the ventricle may not have actually contracted. If the V-sense event is too close to the A-pace event, a ventricular safety pace (VSP) stimulation pulse is given to the ventricle, in case the V-sense was actually an indication of a premature ventricular contraction, which might continue as ventricular tachycardia.
In many patients, it would be desirable to wait until closer in time to the expected time of a natural ventricular contraction. However, waiting too long would put the VSP pulse at about the same point in time as the T-wave, which would be undesirable, as pacing in the middle of the T-wave may cause an arrhythmia. The VSP pulse is given because of a premature V-sense, which is believed to not be an indication of an actual ventricular contraction. If the V-sense reflected a real ventricular contraction, there would be nothing seen from the ventricle until the next natural event. Therefore, waiting a long period would gain nothing. If the V-sense was an over-sense, then waiting for the V-sense reflecting an actual ventricular contraction would require waiting too long, putting any required V-pace too close to the T-wave. Thus, in this situation, while it is not known that the V-sense reflected an actual premature ventricular contraction, it is desirable that the ventricle contract. Therefore the VSP pulse will be generated to ensure that the ventricle contracts.
When a pacemaker is operated in DDD mode, the atrium is paced in the absence of a sensed natural event. After the A-pace, there is a time period, a trigger window, within which a V-sense may be detected. If a V-sense is detected during this window, then a VSP pulse will be scheduled, at the end of the VSP timing window or interval.
In one example, where a desired pacing rate of about 120 beats per minute is desired, the VSP, if it is to occur at all, will be scheduled at about 60 milliseconds after the A-pace. In the example where a slower desired pacing rate of about 60 beats per minute is desired, the VSP, if it is to occur at all, is scheduled at about 110 milliseconds after the A-pace. The VSP is normally scheduled no longer than about 80 milliseconds after the V-sense, to avoid being too close to the T-wave. In the absence of any V-sense event within the trigger window after the A-pace, the next scheduled V-pace would not normally occur for a longer period, for example, about 150 milliseconds. This interval from the A-pace to the V-pace can be based on the PAV interval.
In a paced, cardiac cycle, there may be three blanking periods where the pacemaker is unable to sense arrhythmias. The first blanking period follows the A-pace. The second blanking period follows a V-sense, as it is undesirable for the pacing device to double count the V-sense event. The third blanking period follows the V-pace. Thus, if there is a ventricular arrhythmia occurring at a fast rate, the pacing device may see only every other beat, resembling a normal heart beat. This is because every other beat may lie within a blanking interval. It would be desirable to have an improved time window for detecting arrhythmias. In particular, it would be desirable to have at least half of the window between ventricular events available for detecting arrhythmias, even at high pacing rates.
The present invention provides improved methods for cardiac pacing that may find particular use in pacing situations having high pacing rates that would otherwise have substantially shortened windows for detecting arrhythmias, and/or pacing situations forced to accept slow pacing rates to maintain long windows for detecting arrhythmias. The present invention may be described with respect to a cardiac pacing cycle proceeding from a first atrial pace (A-pace) event, followed by a first ventricular pace (V-pace) event, followed by a second A-pace event, followed by a second V-pace event, followed by further A-pace and V-pace events. The A-pace event can be followed by an A-pace blanking interval which in turn is followed by a trigger zone. During the trigger zone, the pacemaker is able to detect V-sense events. The cardiac cycle also includes an atrial to ventricular (A2V) ventricular safety pace (VSP) timing interval, which can begin at the A-pace event. If a V-sense event is detected during the trigger zone, a VSP pace can be generated at the end of the A2V VSP timing window.
The cardiac cycle further includes a V-pace blanking interval following the V-pace event, and a PAV interval giving the scheduled interval between an A-pace event and the following V-pace event. Finally, the cardiac cycle may be characterized by a cardiac overall pacing interval, the ventricular to ventricular (V2V) interval, giving the time from one ventricular event until the next ventricular event.
The present invention may be used to avoid or substantially reduce events where the time available for detecting arrhythmias would otherwise be less than half the V2V interval, or half the overall cardiac cycle interval. This may otherwise be a problem, where the blanking intervals, which are substantially fixed in length, take over half of the V2V interval as the V2V interval decreases, as may occur during high physiological activity periods.
One method according to the present invention reduces the length of the A2V trigger zone in response to recent occurrences of VSP paces. The trigger zone interval length may be decreased in response to a recent higher frequency of VSP paces, and increased in response to a decrease in recent VSP paces. The trigger zone may be decreased, such that the end of the trigger zone occurs substantially prior to the end of the A2V VSP interval. Another method according to the present invention adjusts the overall pacing rate or pacing interval in response to the recent history of VSP events. The overall cycle interval can be increased in response to a recent history of VSP events, and decreased in response to a lack of recent VSP events. The overall pacing rate may thus be decreased below that otherwise called for in the presence of VSP events in order to lengthen the ventricular to atrial (V2A) window for detecting arrhythmias. In yet another method according to the present invention, the arterial to ventricle interval (PAV) may be decreased in response to the recent occurrence of VSPs, while leaving the overall cycle interval unchanged. This will also increase the arrhythmia detection window between the V-pace and the A-pace events.
One method according to the present invention includes increasing the overall pacing interval responsive to detecting V-sense events, and decreasing the overall pacing interval responsive to recent overall pacing intervals having a minimum V2V interval ending in a ventricular pace that is long enough so as to not interfere with arrhythmia detection. Another method according to the present invention includes waiting for detection of a ventricular event, either a V-sense or a V-pace event. Upon detection of the ventricular event, if the ventricular event is a V-sense detected during the trigger zone, the overall pacing interval is increased. Upon detection of the ventricular event, if the ventricular event is either not a V-sense, or the ventricular event is detected outside the trigger zone, and if a recent history indicates all recent pacing intervals have at least a DV2V interval, then the overall pacing interval is decreased. The DV2V interval represents the minimum V2V interval ending in a ventricular pace that is long enough so as to not interfere with arrhythmia detection.
In yet another method according to the present invention, the trigger window interval is increased in response to detecting recent VSP events, and the trigger window interval is decreased in response to detecting VSP events. The detecting steps can include detecting N VSP events in the previous M time slots, where N is an integer and M has units of time.
In still another method according to the present invention, the method includes waiting for detection of a ventricular event, either a V-sense or a V-pace. Upon detection of the ventricular event, if the most recent V2V interval is not less than the DV2V interval, and all of the most recent N V2V intervals were greater than or equal to DV2V, then the PAV value is increased, but not above an upper limit. Upon detection of the ventricular event, if the most recent V2V interval is less than the DV2V interval, then the PAV value is decreased, but not below a lower limit. The PAV value is then used to schedule the next V-pace.
The present invention further includes computer programs for executing methods described herein. The present invention includes pacemakers containing programs and executing those programs for executing methods described in the present application.